Electronics practically all measure flat in frequency response. Even with frequency response: Have you ever had a 20 band equalizer with a 20K setting and moved it down -2db? Do you think you can hear that? Most of us here do not hear above 14K. Of course, if you did 1K we would hear that quite a bit.
Your statement above shows that you have spent 0 effort trying to understand how the LSA amp would behave with a real load, and especially with the ops speakers. Instead you insist on grasping onto things like solder or connectors, or ... and no they have not wasted time on a double blind test, because that is just what it would be, a waste. I think if you search long and hard, though you may find one on capacitors in a cross-over, but the result will not be to your liking.
I have explained, in excruciating detail, how electronics, in this specific case, will absolutely effect listening impressions. I have brought you to the water, but I cannot make you drink.
I believe Nelson Pass was just being polite. Anything he has put down to paper (i.e. on the record) has been backed up by actual data, i.e. he tackled the potential for frequency response variance with cables. He has never, to my knowledge, given credence to "tweaks", but he understands his customer base, so he knows not to put them down either. It would not be good for business.
In terms of how distortion contributes to perceptions of sound, this is just every day common knowledge now (or should be if you are claiming to be an amp expert) including what we perceive as pleasant, unpleasant, how it may impact loudness impression, masking functions, etc. As always Google.